Forward Link Power Control

ABSTRACT

A method of forward link power control in a communications system comprises: grouping a plurality of user terminals into a plurality of groups, at least one of which comprises more than one of the user terminals; for each said group, determining a corresponding forward link power level required to satisfy an aggregate demand of the group; and assigning one or more forward link carriers to the group, with the corresponding power level.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for powercontrol in a forward link of a communication system, particularly butnot exclusively a satellite communications system.

BACKGROUND OF THE INVENTION

Many satellite communications systems have Adaptive Coding andModulation (ACM) that aim to maximize the throughput of a forward link(i.e. for transmission to a user terminal (UT)). An example system isdisclosed in US2014/0056335 A1.

FIG. 1 depicts a satellite communication system comprising a groundstation 1 that transmits uplink signals UL to a satellite 2, whichtransmits corresponding downlink signals DL in one or more beams. Aplurality of UTs (User Terminals) 6 in a beam 5 are served by twoforward carriers FC1 and FC2. The forward carriers FC1, FC2 are sharedbetween multiple user terminals 6. In a typical satellite communicationnetwork both the forward carriers FC1, FC2 will operate with a fixedEIRP (Effective Isotropic Radiated Power).

Consider an example where a UT 6 is making a voice overinternet-protocol (VOIP), and is in an area where there is excellentsignal strength. In a conventional ACM method the UT 6 will report itslink conditions and the network will adapt the code rate and modulationso that the user can achieve maximum data rate. However, the UT 6 onlyrequires a sufficient data rate to make a VOIP call, whereas the maximumdata rate may only be required if the UT 6 is streaming real time data.Hence, the conventional approach to ACM may not give the optimum overallsystem performance.

SUMMARY OF THE INVENTION

Aspects of the present invention are defined by the accompanying claims.

Embodiments of the invention include a method to optimise the systemperformance for given aggregate satellite power in the forwarddirection. This method may maximise total system throughput, rather thanper user throughput in cases where the forward link is shared with aplurality of users.

Embodiments of the invention may use an algorithm that overcomes theconventional mismatch of requirements by adjusting the forward carrierEIRP.

The method may maintain equilibrium between ACM and optimisation ofsystem capacity, for example by grouping of user terminals based ondemand and/or geographic location, optimising the forward link powercontrol such that the demand for each group is met, and balancing thetotal power available to optimise the link per user terminal group.

Optimising the forward link power may involve one or more of thefollowing benefits. First, power distribution is no longer fixed so thatuser terminal groups with higher demand can be serviced with a higherpower. Second, some user links may be operated with a lower power thanin a conventional ACM method, which leads to reduction in interference,further improving system performance.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the present invention will now be described withreference to the accompanying drawings, in which:

FIG. 1 is a diagram of a satellite communication system using aconventional ACM method;

FIG. 2 is a diagram of a satellite communication system using an ACMmethod according to an embodiment of the invention;

FIG. 3 is a flowchart of the ACM method in the embodiment; and

FIG. 4 is a diagram illustrating a specific use example of theembodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

Apparatus for use in an embodiment of the invention is shown in FIG. 2,in which similar parts to those in FIG. 1 carry the same referencenumerals.

In the following discussion, the term ‘carrier’ may refer to a frequencychannel having a predetermined bandwidth. The carrier may be sharedbetween different UTs 6, for example using time slots in a TDMA systemor spreading codes in a CDMA system.

The ground station 1 is arranged to maintain a record of demand from theUTs 6, for example from forward data queue length and/or demandrequirements received from the UTs 6. The ground station 1 and/or thesatellite 2 are able to adjust independently the power (e.g. EIRP) ofeach forward link carrier, for example for the forward carriers FC1,FC2.

The ground station periodically runs the algorithm shown in FIG. 3 toachieve optimised system capacity. Step S1 is a trigger event to startthe method. The trigger can be a periodic time to wake up the groundstation 1 to run the algorithm or an event such as addition of a new UTto the pool. The next step S2 is to group the UTs 6. The ground station1 monitors the forward data queue and any pre-negotiated demandparameters for a particular UT to calculate the user terminal demand inthe forward link. The ground station 1 then sorts the UTs 6 based ontheir traffic demand. The grouping is then done based on segregating theUTs into groups 6.1 and 6.2. There may be more than 2 groups, butpreferably the number of groups is limited to a maximum predefinedvalue.

In Step S3 the ground station 1 calculates the maximum EIRP neededwithin a group 6.1, 6.2 to satisfy the demands of every UT within thatgroup.

In step S4 the power control function of the satellite 2 and/or theground station 1 is used to adjust the EIRP for each forward link FC1,FC2, preferably subject to the following constraints.

Constraints

Let there be a maximum of θ_(max) groups per beam 5 and let up_(iθj) bethe EIRP required to achieve a data rate dr_(iθj) for user_(iθj) wherei=i^(th) user and θj=j^(th) user group.

Constraint 1: Total Beam Power

${bp}_{n} = {{\sum\limits_{j = 0}^{\theta \; \max}{up}_{\theta \; {jmin}}} \leq {{Max}\mspace{11mu} E\; I\; R\; P\mspace{14mu} {per}\mspace{14mu} {Beam}\mspace{14mu} 5}}$

Where up_(θjmin) is the minimum power required in the j^(th) user groupto attain max(dr_(iθj)).

Constraint 2: Aggregate EIRP

${power}_{total} = {{\sum\limits_{j = 0}^{\theta \; \max}{bp}_{n}} \leq {{Aggregate}\mspace{14mu} E\; I\; R\; P\mspace{14mu} {for}\mspace{14mu} {the}\mspace{14mu} {satellite}\mspace{14mu} 2}}$

Algorithm

An example of the method of FIG. 3 will now be described as analgorithm.

Start S1:

-   -   init_qos_(i)=Read QoS per User    -   cno_(current) ^(i)=Read CNo for User “i”    -   capacity_(carrier) ^(i)=lookupdatarate(min(cno_(current) ^(i)),        ue_type)    -   Look-up Table for Data-rate or carrier to Noise Ratio

For each User Equipment Type the network will store a look up table amapping of the CNo and Datarate, for example as shown below.

-   -   LookupDataRate(Look up value, UT Type)

Type Sub Type C/No Data Rate T5X4 L8 51.5 110.7 T5X4 L7 52.6 135 T5X4 L653.75 162 T5X4 L5 54.72 190.8 T5X4 L4 55.77 220.5 T5X4 L3 56.83 250.2T5X16 L2 57.35 270 T5X16 L1 58.43 315 T5X16 R 59.53 360 T5X16 H1 60.50400.5 T5X16 H2 61.50 441 T5X16 H3 62.48 486 T5X16 H4 63.74 531 T5X16 H564.80 559.8 T5X16 H6 65.45 576 T5X64 H2 66.25 634.4 T5X64 H3 67.45 702T5X64 H4 68.64 767 T5X64 H5 69.75 819 T5X64 H6 70.95 858

Where Type indicates the modulation type (the number after ‘X’indicating the number of possible modulation symbols), and Sub Typeindicates the FEC coding rate.

GROUP Users S2:

  cno_(required) ^(i) = lookupdatarate(init_qos_(i), ue_(type))deficit^(i) = cno_(current) ^(i) − cno_(required) ^(i) sort_user_(i) =sorthightolow(deficit) Let qos_(i) = init_qos_(i) of sort_user_(i)${cum}^{i} = {\sum\limits_{k = 0}^{k = i}{qos}_{i}}$ n = 0 Loop1:  i =0  loop2:  If(cum^(i)/capacity_(carrier) ^(n) > 0):   capacity_(group)^(n) = cum^(i)   user_(n) ^(i) = sort_user_(i)   Increment i  Else:  Exit loop1  End of If:  End of loop2:  total_carriers = n  Increment nEnd of loop1:

Calculate EIRP S3:

groupcno_(required) ^(n) = lookupdatarate(capacity_(group) ^(n),ue_(type))   user_(n) ^(least)_capable = Read CNo for least capable UTin Group ′n′ cno_deficit^(n) = user_(n) ^(least)_capable −groupcno_(required) ^(n) For all Carriers If cno_deficit^(n) = 0 thendon't do anything for this carrier If cno_deficit^(n) > 0: Add carrierto the list ‘dec_power’ as the carier power has to be decreased Else:Add carrier to the list ‘inc_power’ as the carrier power has to beincreased Let D = number of carriers that have to be decreased in power,B be the total number of carriers that have to be increased in power andC be the total number of carriers that do not need any power changeLoop: For each carrier (n = 1 to D) that needs power decrease   eirp^(n)= Read EIRP of carrier ′n′   user_(n) ^(least)_capable = Read CNo forleast capable UE in Group ′n′   delta_cno^(n) = user_(n)^(least)_capable − groupcno_(required) ^(n)   If delta_cno^(n) = 0   D =D − 1 Remove carrier for list of acrriers to be reduced Else:  power_adjust^(n) = delta_cno^(n) * α where α is the convergence factor< 1   Adjust_EIRP (carrier^(n), − power_(adjust) ^(n)) For each carrier(n = 1 to B) that need power increase   eirp^(n) = Read EIRP of carrier′n′   ${current\_ power} = {\sum\limits_{j = 0}^{j = {D + B + C}}{eirp}^{j}}$  available_power = Max EIRP per Beam − current_power   power_adjust^(n)= available_power/B   Adjust_EIRP ( carrier^(n), power_(adjust) ^(n))Decrement D If D = 0 Exit Loop: End of Loop:

Adjust EIRP S4:

Adjust_EIRP ( carrier^(n), power_(adjust) ^(n)) If power_(adjust) ^(n) <Satellite_power_Adjust_Threshold   satellite_carrier_eirp^(n) =satellite_carrier_eirp^(n) + power_adjust^(n) Else power_(adjust) ^(n) >Satellite_power_Adjust_Threshold   satellite_carrier_eirp^(n)       =satellite_carrier_eirp^(n)       + Satellite_power_Adjust_Threshold  power_adjust^(n)       = power_adjust^(n) −Satellite_power_Adjust_Threshold   SAS_carrier_eirp^(n) =SAS_carrier_eirp^(n) + power_adjust^(n)

End S5:

End Algorithm

Specific Use Example

A specific use example will now be described with reference to FIG. 4,which shows a beam pattern of a satellite network utilising afour-colour re-use scheme. In other words, carriers are assigned intofour groups, with the groups being re-used between different beamshaving a minimum separation.

In an example, there are 3 users in each of Beam A and Beam B, withsymmetric demands. For ease of link analysis, the beam isolation at allpoints is assumed to be the same. The user demands are as shown in Table1 below.

TABLE 1 User Demands Max QoS kbps User Service Type requested User 1Aand 1B Voice Call 24 User 2A and 2B Streaming 128 User 3A and 3B HighQuality Video 858

The system will assign two full carriers F1 and F2 to users 3B and 3Arespectively as they demand the maximum throughput that can be achievedby the system. Users 1A and 2A can be served on the same frequency astheir maximum aggregate rate is 152 kbps which can be served with onecarrier.

As the frequencies can be re-used, Users 1A and 1B are assignedfrequency F1 and Users 2A and 2B are assigned frequency F2. In aconventional scenario the network will set both carriers with the sameEIRP, for example 41.5 dBW. The conventional ACM allocates the bestpossible modulation and code rate. Table 2 details the link analysis forsuch a situation.

TABLE 2 Link analysis for all carriers with equal EIRP Max QoS EIRP CCIEIRP C/No Throughput Beam Frequency Users kbps dBW dBW dBHz Code Ratekbps A F1 1A-F1 24 41.5 41.5 64.5 T5X16-H4 531 2A-F1 128 F2 3A-F2 85641.5 41.5 64.5 T5X16-H4 531 B F2 1B-F2 24 41.5 41.5 64.5 T5X16-H4 5312B-F2 128 F1 3B-F1 856 41.5 41.5 64.5 T5X16-H4 531

The total power used by the two beams A and B is 47.5 dBW and the usefulaggregate Data Rate is 1366 kbps.

In contrast, an implementation of the algorithm in an embodiment of theinvention will now be described. The network has the information thatUsers 1A and 1B need a maximum of 24 kbps (IP voice) and Users 2A and 2Bneed a maximum of 128 kbps streaming rate i.e. an aggregate maximum of152 kbps. Therefore the network can lower the power for those carriers.On the other hand users 3A and 3B need 858 kbps, which they cannotachieve. Therefore, the network will have to increase the power. Thenetwork optimises management of the co-channel interference and therequired data rate, and adjusts the carrier powers as shown below,maintaining the same total power per beam.

TABLE 3 Link analysis for all carriers with optimised EIRP Max QoS EIRPCCI EIRP C/No Throughput Beam Frequency Users kbps dBW dBW dBHz CodeRate kbps A F1 1A-F1 24 31.5 44 54.4 T5X4-L6 162 2A-F1 128 F2 3A-F2 85644 31.5 67.6 T5X64-H3 702 B F2 1B-F2 24 31.5 44 54.4 T5X4-L6 162 2B-F2128 F1 3B-F1 856 44 31.5 67.6 T5X64-H3 702

The total power used in the total beams is almost the same as in theconventional case (the embodiment requires 0.25 dB less). The total datarate achieved is 1708 kbps i.e. 25% increase in total capacity for thetwo beams. In addition, the maximum per-user throughput goes up by 32%from 531 kbps to 702 kbps.

The application of the algorithm in the embodiment to the above examplewill now be described.

Start S1:

Each User signals their Max QoS (Quality of Service):

-   -   1A, 1B Signals 24 kbps    -   2A, 2B signals 128 kbps    -   3A, 3B signals 858 kbps    -   The RAN records the current CNo for reach user. For simplicity        in this example, they all have a CNo of 64.4 dBHz    -   Current max capacity of each current carrier=531 kps

Group Users S2:

The required CNo is obtained from a look up table which is specific to agiven satellite network.

In this case for beam 1 the following are the required CNo

-   -   1A, 1B require 51.5 dBHz    -   2A, 2B require 52.6 dBHz    -   3A, 3B require 71 dBHz

The above steps are shown for Beam A; the same applies to Beam B.

-   -   Calculate deficit    -   1A=64.4−51.5=12.9    -   2A=64.4−52.6=11.8    -   3A=64.4−71.0=−6.6    -   Sort User from Highest to Lowest    -   1A, 2A, 3A    -   Calculate Cumulative Max Qos    -   24, 152, 1010    -   Group based on max capacity of current carrier=531    -   Group 1=1A, 1B with aggregate data requirement of 152    -   Group 2=3A with aggregate data requirement of 852

Calculate EIRP S3:

The required CNo for each group is obtained from a look up table whichis specific to a given satellite network.

-   -   Group 1=53.75    -   Group 2=71    -   Calculate deficit CNo    -   Group 1=64.4−53.75=10.75    -   Group 2=64.4−71=−6.6    -   Let F1 be the frequency for Group 1 and F2 for Group 2

Therefore, power of F1 has to be reduced whereas the power of F2 has tobe increased.

Applying the EIRP control algorithm of the embodiment:

TABLE 4 Example 1: alpha = 0.5 slower convergence EIRP EIRP CurrentPower Total F2 F1 CNo DeltaCNo Delta * adjust Power dBW dBW dBHz dB 0.5dB dB dBW 41.5 41.5 64.5 10.75 5.4 5.0 44.5 44 36.5 59.13 5.38 2.7 3.044.5 44 33.5 56.54 2.79 1.4 1.0 44.5 44 32.5 55.39 1.64 0.8 1.0 44.5 4431.5 54.43 0.68 0.3 0.0 44.5

TABLE 5 Example 2: alpha = 0.8 fast convergence EIRP EIRP Current PowerTotal F2 F1 CNo DeltaCNo Delta * adjust Power dBW dBW dBHz dB 0.8 dB dBdBW 41.5 41.5 64.5 10.75 8.6 9.0 44.5 44 32.5 55.39 1.64 1.1 1.0 44.5 4431.5 54.43 0.68 0.5 0.0 44.5

Alternative Embodiments

Alternative embodiments of the invention may be envisaged, which maynevertheless fall within the scope of the accompanying claims.

1. A method of forward link power control in a wireless communicationsystem, the method comprising: a. grouping a plurality of user terminalsinto a plurality of groups, at least one of which comprises more thanone of the user terminals; b. for each said group, determining acorresponding forward link power level required to satisfy an aggregatedemand of the group, and assigning one or more forward link carriers tothe group, with the corresponding power level.
 2. The method of claim 1,wherein the user terminals are grouped by forward link demand.
 3. Themethod of claim 2, wherein the demand is determined from forward dataqueue length.
 4. The method of claim 2, wherein the demand is determinedfrom demand requirements received from the user terminals.
 5. The methodof claim 2, wherein at least one of the groups comprises a single saiduser terminal having a higher demand than other ones of the userterminals.
 6. The method claim 1, wherein the users are grouped bygeographic location.
 7. The method of claim 1, wherein the wirelesscommunication system is a satellite communications system.
 8. The methodof claim 7, wherein the user terminals are arranged in a plurality ofbeams in which the carriers may be re-used subject to a minimum re-usedistance.
 9. The method of claim 1, wherein the number of groups islimited to a predetermined maximum.
 10. The method of claim 1, whereinthe forward link power levels are constrained by a maximum aggregatepower level.
 11. The method of claim 1, wherein the power levelcomprises an EIRP.
 12. The method of claim 1, wherein the carrierscomprise frequency channels.
 13. The method of claim 1, includingassigning a corresponding coding and/or modulation rate to each of theforward link carriers.
 14. The method of claim 1, including transmittingto the user terminals using the forward link carriers with thecorresponding power levels.
 15. A wireless communication systemcomprising: a plurality of user terminals; and a transmitter arranged totransmit wireless signals to the plurality of user terminals over aforward link; wherein the plurality of user terminals are grouped into aplurality of groups, at least one of which comprises more than one ofthe user terminals; for each said group, a corresponding forward linkpower level required to satisfy an aggregate demand of the group isdetermined, and one or more forward link carriers are assigned to thegroup, with the corresponding power level.
 16. A non-transient computerreadable medium containing program instructions for causing a computerto: a. group a plurality of wireless user terminals into a plurality ofgroups, at least one of which comprises more than one of the userterminals; b. for each said group, determine a corresponding forwardlink power level required to satisfy an aggregate demand of the group,and c. assign one or more forward link carriers to the group, with thecorresponding power level.